Skip building wasm-opt-sys when it has already been built

[clearloop]

the build.rs of wasm-opt-sys takes too much time

[brson]

The lack of caching in the C++ build step is the big disadvantage of how we're building binaryen using only the cc crate and without cmake. Any time the wasm-opt-sys build script is invoked it will rebuild all the C++ files.

cc explicitly doesn't support any kind of caching or incremental compilation - it's out of scope for that crate.

Fixing this is possible though not trivial.

We could just use cmake but that imposes multiple new external dependencies that are less likely to be available than a C++ compiler, the reason I decided to use cc for the build.

We could also attempt to add a caching layer on top of the cc crate. Even an imperfect implementation would probably be usable for the purpose of not rebuilding all of binaryen. Other projects might want this as well.

My preference is to the latter. It's something I would be interested in doing, but I'll need to see whether it's in scope of the funding for this project.

[brson]

I've thought a bit about how to add a caching layer on top of the cc crate, and I have not come up with a plan that I think will obviously work. The big obstacle is that we also have to interop with the cxx_build crate, which is itself a wrapper around the cc::Build constructor; and I don't see a way to inject any behavior between cxx_build and cc::Build.

cxx_build is pretty simple though so one possibility is to just copy that code and duplicate its behavior.

[brson]

I've thought about this some more, and think there may be a way to cache cc build results even with cxx_build, at least with the limited features that wasm-opt uses. I've sketched out an API and docs here:

https://github.com/brson/wasm-opt-rs/blob/cache/components/wasm-opt-sys/cc_cache.rs

I am feeling wary of actually attempting this since there are so many unknown ways caching a C++ build can go wrong. But idk maybe we'll try it.

[brson]

One thing to note is that even if we skip building some C++ files, I don't see any way to avoid rerunning ar to create the library rustc needs to link to, and the time to run ar is not insignificant.

[brson]

There is another potential approach here: sscache already does build caching for rust and C++. It could be possible to automatically acquire sccache and configure it as part of the build script.